class Solution {
    vector<vector<bool>> vis;
    int m, n;

public:
    int numIslands(vector<vector<char>>& grid) {
        m = grid.size(), n = grid[0].size();
        vis = vector<vector<bool>>(m, vector<bool>(n));
        int ret = 0;
        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++) {
                if (!vis[i][j] && grid[i][j] == '1') {
                    ret++;
                    dfs(grid, i, j);
                }
            }
        return ret;
    }
    int dx[4] = {0, 0, -1, 1};
    int dy[4] = {1, -1, 0, 0};
    void dfs(vector<vector<char>>& grid, int i, int j) {
        vis[i][j] = true;
        for (int k = 0; k < 4; k++) {
            int x = i + dx[k], y = j + dy[k];
            if (x >= 0 && x < m && y >= 0 && y < n && !vis[x][y] &&
                grid[x][y] == '1') {
                dfs(grid, x, y);
            }
        }
    }
};
